Reading · Science Crosstalk: where science and communication collide

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August is National Immunization Awareness Month (NIAM), a whole month dedicated to raising awareness to one of public health’s biggest success stories.

Say the word “vaccine“ out loud, and watch what happens. Some people relax. Others tense up. Most land somewhere in between, with a bunch of lingering questions and half-remembered facts. Let’s skip the noise and get straight to the biology.

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Vaccines Give Your Immune System a Practice Run

Think of your immune system as a highly trained security team. The moment an unfamiliar pathogen, like a virus or a bacterium, slips past your defenses, that team scrambles to figure out the threat. Sizing up a total stranger takes time. While your immune system is still gathering intel, you’re the one getting sick.

Vaccines close that gap. They introduce a harmless version or fragment of the pathogen — enough to be recognizable, not enough to make you ill. Your immune cells study it, learn to how defeat it, and file it away for later in the form of memory T, memory B cells and antibody. So, when the real pathogen eventually shows up, your body isn’t starting from scratch. It already knows exactly what to do.

Current types of vaccines approved in the U.S. and how they work:

Live-Attenuated Vaccines
Use weakened versions of pathogens so they can still replicate just enough to train the immune system but not cause illness. Often require fewer doses and provide long-lasting protection. Examples include MMR (measles, mumps, rubella), varicella (chickenpox), and yellow fever vaccines.

Inactivated (Killed) Vaccines
Use pathogens that have been inactivated (killed) so they cannot replicate. They provide a safe way to stimulate immunity, though multiple doses may be needed. Examples include inactivated flu shots, inactivated polio, hepatitis A, and rabies vaccines.

mRNA Vaccines
Use messenger RNA to instruct your cells to produce a harmless piece of a virus (i.e. the spike protein for SARS-CoV-2) triggering an immune response. The mRNA itself doesn’t enter the nucleus or alter DNA. Examples include Pfizer‑BioNTech (Comirnaty) and Moderna (Spikevax, mNexspike) COVID‑19 vaccines.

Future promise: Researchers are exploring mRNA technology for treating pancreatic cancer and cystic fibrosis, because it can deliver genetic instructions to correct or fight disease at the cellular level.

FUN FACT. Did you know that every cell in your body makes and uses mRNA? It’s true! mRNA is short for messenger RNA and every cell in your body makes and uses mRNA as the recipe or set of instructions to make all the proteins in your body. mRNA is not foreign or new. Our bodies have been making and using mRNA since the very beginning of human life!

Viral Vector Vaccines
Use a harmless, modified virus (vector) to deliver genetic instructions to your cells so they produce a target protein from the pathogen. That protein triggers immunity. An example: the Ebola vaccine (Ervebo), which uses a live but weakened viral vector.

Protein Subunit and Recombinant Vaccines
Contain purified pieces of the pathogen, like proteins, that train the immune system without using live components. An example is the Novavax COVID‑19 vaccine (Nuvaxovid), which uses recombinant proteins with an adjuvant to enhance the immune response.

Conjugate and Polysaccharide (Subunit) Vaccines
Use specific parts of bacteria, like the polysaccharide capsule, often linked (conjugated) to a carrier protein to enhance the immune response. Examples include Haemophilus influenzae type b (Hib), pneumococcal, and meningococcal conjugate vaccines.

Toxoid Vaccines
Contain inactivated toxins produced by bacteria, rather than the bacteria itself. They teach the immune system to neutralize the toxins. Examples include diphtheria and tetanus found in TDaP (tetanus, diphtheria and acellular pertussis) vaccine.

Vaccines Don’t Overwhelm Your Immune System

Some people worry that vaccines “overload” the immune system. The truth? Your immune system is built for diversity. Every B cell and T cell in your body has a unique specificity—it’s trained to recognize a different piece of a pathogen. Collectively, these cells offer tremendous diversity, capable of responding to millions of different threats.

Think of the immune system in your body like a hospital. There are many different types of specialists, such as cardiologists, pulmonologists, and neurologists, each trained for a specific field of medicine. If you come in with a heart problem, you’ll see the cardiologist (heart doctor), not the pulmonologist (lung doctor) or neurologist (brain doctor). Similarly, when a vaccine introduces a piece of a virus or bacteria, only the immune cells with the right “specialty” respond. The rest stay on standby waiting for the piece of pathogen they recognize.

Protection Without the Risk

After vaccination, your immune system has memory cells and antibodies ready to act. If you encounter the real pathogen, those memory cells recognize it instantly and mount a rapid defense, often before you even feel sick.

Vaccines don’t overwhelm your immune system. Vaccines guide immunity to make it smarter, faster, and better prepared.

FAQ About Vaccines

1. Can vaccines cause the disease they protect against?
No. Vaccines use inactivated pathogens, weakened forms, or just pieces of the pathogen (like proteins). They give your body a practice run to learn what the pathogen looks like to build defenses like memory immune cells and antibodies.

2. Why do we need additional doses (aka boosters) for some vaccines?
Over time, antibodies decrease and immune memory needs a nudge. Additional booster doses remind your immune system what the pathogen looks like, resulting in a refresher for immune memory cells and an increase in antibody production to keep your defenses strong.

3. Do vaccines weaken the immune system?
No. Vaccines actually strengthen – not weaken – your immune system. Vaccines train your immune system without overwhelming it. Your immune system is designed to handle thousands of challenges at once and vaccines guide it safely with harmless versions of a pathogen to build defenses.

4. Why do some vaccines protect for life while others need updates?
It depends on the pathogen and the formulation of the vaccine. Some viruses mutate quickly (like the influenza and SARS-CoV-2, the virus that causes COVID), so vaccines need annual updates. Other pathogens (like measles and polio) stay relatively stable, so one vaccine can last decades. Some formulations of vaccines, like live-attenuated, result in a robust immune response on their own while subunit and other formulations may require additional doses to keep protection at its peak.

5. Can my immune system handle multiple vaccines at once?
Yes! Your immune system encounters countless microbes every day. The number of antigens in vaccines is tiny compared to what it handles through every day life. Vaccines train your immune system without overwhelming it.